Field of the Invention
This invention relates to an optical waveguide element and an optical modulator used in the field of optical communication and optical instrumentation. More particularly, the invention relates to an optical waveguide element having a ridge structure, and to an optical modulator using this optical waveguide element.
Description of Related Art
As the use of the Internet spreads, the amount of data communicated is rapidly increasing, making the optical fiber communication very important. In the optical fiber communication, electric signals are converted into optical signals, and the optical signals are transmitted through optical fibers. The optical fiber communication is characterized in that the signals are transmitted in the broad band, with a small loss, and are not affected by noise.
Known as systems for converting electric signals into optical signals are the direct modulation system using a semiconductor laser and the external modulation system using optical modulators. The direct modulation system need not use optical modulators and its running cost is low, but cannot achieve high-speed modulation. This is why the external modulation system is used in high-speed, long-distance data communication.
In practically used optical modulators, an optical waveguide is formed by titanium (Ti) diffusion in the vicinity of a surface of a single-crystal lithium niobate substrate. High-speed optical modulators of 40 Gb/s or more are commercially available. However, these high-speed optical modulators have the drawback of having a length as long as approximately 10 cm.
Japanese Patent Application Laid-Open Nos. 2014-006348, 2014-142411 and 2015-014716 disclose Mach-Zehender optical modulators, each having a sapphire single-crystal substrate and a lithium niobate film epitaxially formed on the substrate, 2 μm or less thick, c-axis orientated and shaped like a ridge. Any optical modulator that has a lithium niobate film is much smaller than, and can be driven at a lower voltage than, the optical modulator having a lithium niobate signal-crystal substrate.
FIG. 30 is a sectional view of a ridge-shaped optical waveguide element 400 of conventional type. The optical waveguide element 400 includes a substrate 1 and a waveguide layer 2 formed on the substrate 1. The waveguide layer 2 is made of lithium niobate and has a ridge part 3. The ridge part 3 is a projection having ridge width W and thickness T1. Slab parts 4 are provided at the both sides of the ridge part 3, respectively, and have thickness T2 (<T1).
The conventional optical waveguide element 400 is disadvantageous in that the propagation loss is large in TM fundamental mode. The reason why a propagation loss of the element 400 is large in TM fundamental mode is the coupling of TM fundamental mode to TE higher-order mode. In TE higher-order mode, light propagates to, for example, the slab parts 4 located outside the ridge part 3, not restricted by the ridge part 3. Hence, the propagation loss in TM fundamental mode can be reduced by suppressing the coupling of TM fundamental mode to TE higher-order mode.
The conventional optical waveguide element 400 is disadvantageous, also in that the propagation loss in TM fundamental mode increases if the ridge width W decreases.
Moreover, the propagation loss abruptly increases even if the shape of the ridge part 3 differs only a little from the design shape. In such a case, the propagation loss may increase due to a variation in manufacturing processes.